Like wood, wood composites are susceptible to biological attack from wood destroying fungi and insects. They are also very vulnerable to fire. A wide range of wood preservatives and fire retardants has been developed for wood treatment. However, many preservatives are high in mammalian toxicity and have adverse effects on the environment. Some may also be detrimental to the wood itself.
Typically, sodium borates have efficacy as preservatives and fire retardants for wood products. They are also low in cost and mammalian toxicity, and have a minimum environmental impact. Accordingly, many attempts have been made to use borates to pre-treat wood furnish, such as wood wafers, particles and fibres in the manufacture of wood composites such as wafer board and particle board. However, and hitherto, it has been recognized that sodium borates result in poor adhesive bonding, even at very low boron retentions, when phenolformaldehyde resins are used as the binder.
For example, in an article entitled "Compatability of Nonacidic Waterborne Preservatives with Phenol-Formaldehyde Adhesive" by C. B. Vick, R. C. DeGroot and J. Youngquist published in the Feb. 1990 edition of Forest Products Journal, Volume 40, No. 2 p. 16, it was reported that three borate preservatives, namely ammoniacal copper borate, ammoniacal pentaborate and disodium octaborate tetrahydrate resulted in poor particle bonding even at the lowest retention value for the preservatives. Additionally, P. E. Laks and R. D. Palardy reported that hydrated water-soluble borates such as disodium octaborate tetrahydrate interfered with the glue line strength development of phenolic adhesives in an article titled "The Development of Borate-Containing Flake Board", First International Conference on Wood Protection with Diffusable Preservatives, proceedings 47355, Nashville, Tennessee.
Knudson et al in U.S. Pat. No. 4,879,083 issued Nov. 7, 1989 also recognized that the strength of wafer board was reduced to an unacceptable level when disodium octaborate tetrahydrate was used as an additive, but found that if a low solubility borate such as zinc borate or anhydrous borax was used as the preservative, the resulting bond strength was not significantly impaired. By employing a low solubility rate borax such as zinc borate or anhydrous borax in a conventional press having heated press platens, he observed that the amount of borax dissolved by the steam front as it moved through the board during consolidation is such that the resin viscosity is not impaired sufficiently to effect the bond. When, however, a soluble borate was employed as an additive, the strength of the resultant board was found by him to be at an unacceptable level and was believed by him to be due to solubility of water soluble borate on the adhesive.